1. Field of the Invention
The present invention relates to a voltage control oscillator mounted on various kinds of high frequency devices, more specifically, the voltage control oscillator to be operated by switching two frequencies different from each other as the oscillation frequency.
2. Description of the Related Art
The construction of a conventional voltage control oscillator is explained referring to FIG. 4.
In FIG. 4, 1 denotes a voltage control oscillator, which comprises a resonance circuit 2, an oscillation circuit 3, a buffer circuit 4, and a matching circuit 5.
The resonance circuit 2 is a circuit to select either of two frequencies different from each other, and to output the signal resonated by the selected frequency. The specific construction of the resonance circuit 2 is not indicated in a figure. For example, it comprises two resonance devices to be resonated at the resonance frequencies different from each other, and selects and drives either resonance device, and switches two frequency signals to output the switched one. Another resonance circuit is provided with an inductor and a capacitor, regulates the synthesized impedance by regulating the inductance or capacitance generated in these elements, and switches two frequency signals different from each other to output the switched one.
The oscillation circuit 3 comprises a first transistor TR1, and a capacitor C1 to be connected between base-emitter terminals of the first transistor TR1. The emitter terminal of the first transistor TR1 is grounded through a capacitor C2.
The buffer circuit 4 is provided with a second transistor TR2, and a collector terminal of the second transistor TR2 is connected to an output terminal Vout through a capacitor C3. The emitter terminal of the first transistor TR1 is connected to the base terminal of the second transistor TR2 through a coupling capacitor C4. The connection point of the collector terminal of the first transistor TR1 to the emitter terminal of the second transistor TR2 is grounded through a capacitor C5. The capacitor C5 is jointly used by the oscillation circuit 3 and the buffer circuit 4.
The matching circuit 5 is provided with an inductor L1 and a matching capacitor C6, and one end of the inductor L1 is connected to the collector terminal of the second transistor TR2, and the other end of the inductor L1 is connected to a power-supply voltage input terminal Vsc. The connection point of the collector terminal of the second transistor TR2 to the output terminal Vout is grounded through the matching capacitor C6. The connection point of the other end of the inductor L1 to the power-supply voltage input terminal Vsc is grounded through a matching capacitor C7.
In the voltage control oscillator 1, the oscillation frequency to be used can be switched from the frequency fa in the 800 MHz zone to the frequency fb in the 1.6 GHz zone. That is, the output signal of the resonance circuit 2 is received by the base terminal of the first transistor TR1, and amplified by the capacitor C1 and oscillated therefrom. The oscillation signal is received by the base terminal of the second transistor TR2 through the coupling capacitor C4, amplified by the second transistor TR2, and then, received by the matching circuit 5 from the collector terminal of the second transistor TR2, matched by the matching circuit 5, and outputted outside the voltage control oscillator 1 from the output terminal Vout.
However, the conventional voltage control oscillator 1 has the following problems.
When the frequency fa in the 800 MHz zone is switched from the frequency fb in the 1.6 GHz zone as the oscillation frequency of the voltage control oscillator 1, and if fb is close to 2fa which is the higher order harmonic wave of fa, it is difficult to lower the higher order harmonic level of 2fa to obtain the output power of fb, and it is thus difficult to obtain the desired output power.
Thus, the object of the present invention is to provide a voltage control oscillator capable of obtaining the desired output power by lowering the higher order harmonic level.